1. Field of the Invention
The field of art to which the claimed invention pertains is the catalytic cracking of hydrocarbons. More specifically, the claimed invention relates to an FCC process which circulates a sulfur oxide acceptor with the catalyst.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
Due to the ever increasing concern about air pollution, great efforts have been expended in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various industrial operations. One of the most onerous of these pollutants is sulfur dioxide which is present in the stacks of flue gases from various operations. In one such operation, the FCC process, sulfur compounds contained in the hydrocarbon feedstock result in sulfur containing material to be deposited on the FCC catalyst along with the carbonaceous material and thereby cause the generation of sulfur dioxide in the FCC regeneration section when the sulfur is burned off the catalyst along with the carbon deposits. This sulfur dioxide becomes a part of the regenerator flue gas and thus a pollutant when the flue gas eventually finds its way into the atmosphere.
There are many methods known to the art for removal of sulfur dioxide from stack or flue gases. There is, for example, the wet scrubbing process in which the sulfur dioxide reacts with an appropriate reactant contained in an aqueous solution or slurry sprayed into the flue gas, the sulfur thereby being removed from the system as a compound contained in the liquid phase. In another process the flue gas is passed through a fixed solid bed containing a sulfur "acceptor" with which the sulfur dioxide reacts and on which the sulfur is retained in the sulfate form, thereby being removed from the flue gas.
The basic prior art process for removal of sulfur dioxide from FCC flue gas highly pertinent to the present invention is that disclosed in U.S. Pat. No. 4,071,436 to Blanton, Jr., et al. In this process alumina or magnesia particles are in admixture with the FCC catalyst and are circulated therewith throughout the reactor-regenerator circuit. In the regenerator the alumina reacts with sulfur dioxide to form a solid compound, which when circulated to the reactor reacts with hydrocarbons in the feedstock in the reducing environment to release the sulfur. The sulfur is thereby dealt with in the FCC facilities downstream of the reactor section instead of as part of the regenerator flue gas. This reference further states that it is preferred that materials such as sodium not be present in the particulate solid used for removal of the sulfur dioxide.
U.S. Pat. No. 4,153,535 to Vasalos et al discloses the circulation of a sulfur oxide acceptor with FCC catalyst. The acceptor comprises a metallic reactant which ideally may be at least one free or combined metallic element selected from the group consisting of sodium, magnesium and copper. The metallic reactant may be supported on alumina. Suggested methods of incorporating the metallic reactant into the acceptor include impregnation of the support with a water or organic solvent-soluble or dispersible compound or compounds of the metallic reactant or incorporating the metallic reactant with a precursor such as a silica gel or silica-alumina gel.
Other references having similar teachings as the above references but not as relevant or no more relevant to the present invention are U.S. Pat. Nos. 4,153,534 to Vasalos; 4,204,945 to Flanders et al; 4,243,556 to Blanton, Jr.; 4,252,635 to Blanton, Jr.; 4,300,997 to Meguerian et al, and 4,325,811 to Sorrentino. The last mentioned reference also teaches the use of a reducing zone, separate from the reactor and regenerator, in which the sulfur laden acceptor is relieved of sulfur by reduction with hydrogen or a hydrocarbon gas.
The present invention is based on the discovery of a particular acceptor composition and its method of manufacture, which acceptor has unique capabilities with regard to the disposition of sulfur oxides in the regenerator flue gas.